Screws of cortical bone and method of manufacture thereof

ABSTRACT

A screw formed of cortical bone for use in the human body with an implant having a screw hole for receiving at least a portion of a screw therethrough, includes a shaft with a thread along at least a portion of its length. The thread has an outer diameter dimensioned to pass through the screw hole in the implant. The trailing end of the screw is configured to cooperatively engage at least a portion of the screw hole of the implant so as to prevent the screw from linear motion along the mid-longitudinal axis of the shaft in a direction opposite to the direction of insertion when the screw is threaded through the screw hole to attach the implant to a bone portion of the human body. The screw is formed substantially of cortical bone of a single cortical thickness.

RELATED APPLICATION

This is a divisional of application Ser. No. 09/566,055, filed May 5,2000, which claims the benefit of U.S. provisional application No.60/132,671, filed May 5, 1999; all of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is directed generally to screws for orthopedic use inhumans, and specifically to screws made of cortical bone and method ofmanufacture thereof.

2. Description of the Related Art

The mammalian skeleton includes dense structural bone known as corticalbone. In humans, the femur, one of the larger long bones of the body,may have a cortical thickness as great as 6–8 mm. Larger mammals, whichhave been used to provide bone for surgical use in humans, have boneswith cortical thicknesses substantially greater than those found inhumans.

Screws for orthopedic use may be used for a multitude of purposes,including to join separated bone portions, and to attach variousorthopedic implants, such as skeletal plates, to bones. Such screws havecommonly been made of surgical quality metals (e.g. stainless steel,surgical grade titanium, and titanium alloys), ceramics and variousplastics including some that are bioresorbable.

Metal screws typically remain in the body unless explanted by a later,separate operative procedure, and can inter alia potentially irritatetissue proximate the metal screws, shed ions harmful to the body,back-out or loosen causing injury within the body. Metal screws also caninterfere with optimal visualization of the affected area by variousdiagnostic modalities such as x-rays, CAT scans or MRIs. On the otherhand, resorbable screws made of bioresorbable plastic materials whichcan be absorbed by the body over time have had both limitedapplicability and success. Resorbable plastic screws have often beenlimited by insufficient strength, an inability to be formed into a sharpthread-form, an unpredictable absorption rate, an inability to maintainsufficient structural integrity for an adequate period of time, theelicitation of an undesirable inflammatory response, and the potentiallytoxic effects of the degradation products of the material released bythe bioresorption process.

There is therefore a need for a bone screw that combines the advantagesof being sufficiently strong so as to be useful for skeletal fixationand for the attachment of various implants to the human skeleton, thatadditionally is resorbable within the body, and which does not have allthe undesirable qualities encountered with screws of the past.

SUMMARY OF THE INVENTION

In accordance with the present invention, as embodied and broadlydescribed herein, there are provided bone screws made of cortical bonefor orthopedic use in humans. In a preferred embodiment, the bone screwof the present invention is made substantially of cortical bone andcomprises at least a partially threaded shaft that is substantiallysolid, a leading end for introduction of the screw, and an oppositetrailing end. The trailing end is adapted to cooperatively engage aninstrument for turning the screw. The trailing end further comprises ahead configured so as not to be able to pass through the same openingand passageway of an implant, for example, that the threaded shaftportion of the screw is capable of passing through for the purpose ofpreventing the continuing advancement of the screw. The screw head maytake the form of an enlargement having either a greater diameter or agreater maximum dimension than that of the outer diameter of thethreaded shaft portion of the screw. In the alternative, the proximalshaft portion at the trailing end of the screw may have a thread pitchin which the thread crests are closer together than the thread crestsproximate the leading end. In a further refinement of this alternative,it is possible for the outside diameter of the thread to remain constantover the length of the screw shaft and for the thread profile to changefrom a cancellous thread-form for engaging cancellous bone to a machinethread-form at the trailing end to lock the screw at or within anorthopedic implant or at the cortex of a recipient bone itself.

The present invention is directed to an orthopedic bone screw madesubstantially of cortical bone, which is preferably but not necessarilyof human origin. The screw of the present invention is substantiallysolid throughout its shaft portion and has a head portion at itstrailing end configured to resist the further forward advancement of thescrew when the screw is threaded through an appropriately sized opening,such as an opening in an orthopedic implant.

While the present invention is not so limited, the preferred bone ishuman obtained from the diaphyseal region of one of the large tubularbones of the skeleton, such as the femur or from a portion of agenerally intramembraneously formed substantially cortical bone, such asmay be found in the skull.

The present invention is further directed to locks for locking bonescrews to an implant as an element separate from the screw or as part ofthe screw itself. The locks preferably also are made of cortical bonefor locking the screws to an implant. The present invention alsoincludes the combination of screws and locks with an orthopedic implantwhich also is preferably made of cortical bone.

The accompanying drawings, which are incorporated in and constitute apart of this specification, are by way of example only and notlimitation, and illustrate several embodiments of the invention, whichtogether with the description, serve to explain the principles of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a side elevation view of a first embodiment of a bone screw,a screw lock in cross section both made of cortical bone, a partialcross section of an orthopedic implant, and a tool in fragmentary viewfor installing the bone screw and screw lock.

FIG. 1B is a top perspective view of the lock of FIG. 1A.

FIG. 1C is a top plan view of the screw head of the bone screw of FIG.1A.

FIG. 1D is a side elevation view of another embodiment of the bone screwin accordance with the present invention.

FIG. 2A is a side elevation view of a second embodiment of two bonescrews and a screw lock made of cortical bone in accordance with thepresent invention, and a partial cross section of an orthopedic implant.

FIG. 2B is a top plan view of the head of a bone screw in FIG. 2A.

FIG. 3A is a side elevation view of a third embodiment of two bonescrews and a screw lock made of cortical bone, and a partial crosssection of an orthopedic implant.

FIG. 3B is a top plan view of the screw lock of FIG. 3A.

FIG. 4 is a side elevation view of a fourth embodiment of two bonescrews and a screw lock made of cortical bone, and a partial crosssection of an orthopedic implant.

FIG. 5 is a side elevation view of a fifth embodiment of two bone screwsand screw lock made of cortical bone, and a partial cross section of anorthopedic implant.

FIG. 6 is a sixth embodiment of a bone screw and a partial cross sectionof an orthopedic implant each made of cortical bone.

FIG. 7A is a side elevation view of a seventh embodiment of a bone screwand a partial cross section of an orthopedic implant lock made ofcortical bone.

FIG. 7B is a top plan view of the trailing end of the screw in FIG. 7A.

FIGS. 8–10 are side elevation views of an eighth embodiment of a bonescrew, and a screw lock made of cortical bone, and a partial crosssection of an orthopedic implant.

FIG. 11 is a side elevation view of a ninth embodiment of a screw madeof cortical bone.

FIG. 12 is a top plan view of the trailing end of the bone screw made ofcortical bone shown in FIG. 11 with a locking insert therein.

FIG. 13 is perspective view of a locking insert of cortical bone for usewith the bone screw shown in FIGS. 11 and 12.

FIGS. 14 and 15 are a side elevation view and an end view, respectively,of an instrument for inserting the bone screw of FIGS. 11 and 12.

FIG. 16 is a side elevation view in partial cross section of a segmentof the human spine with a spinal fusion implant made of in partial crosssection bone inserted between two adjacent vertebrae, bone screws ofFIG. 5, and a screw lock being installed.

FIG. 17 is a side elevation view in partial cross section of a segmentof the human spine with an orthopedic plate in cross section made ofbone placed across a spinal disc in contact with the adjacent vertebralbodies and screws installed through the orthopedic plate and into theadjacent vertebral bodies.

FIG. 18 is a side elevation view of a fractured long bone with lagscrews made of bone in accordance with the present invention installedalong the fracture to rejoin the long bone.

FIG. 19 is a side elevation view of a long bone with segmentsillustrated in hidden line of cortical bone being removed from thediaphysis by making a longitudinal cut, the segments being used to formbone screws made of cortical bone in accordance with the presentinvention.

FIG. 20 is an enlarged fragmentary end view of FIG. 19.

FIG. 21 is a side elevation view of a tenth embodiment of a bone screwmade of cortical bone removed from the diaphysis of a long bone inaccordance with the present invention.

FIG. 22 is an end view of a segment of cortical bone removed from thediaphysis of a long bone by making a longitudinal cut as illustrated inFIG. 20.

FIG. 23 is a block diagram of the steps of manufacturing and packagingthe bone screws of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS

The following description is intended to be representative only and notlimiting and many variations can be anticipated according to theseteachings, which are included within the scope of the present invention.Reference will now be made in detail to the preferred embodiments ofthis invention, examples of which are illustrated in the accompanyingdrawings.

FIGS. 1A–12, 18, 21, and 22 show various embodiments of screws made ofcortical bone in accordance with the present invention generallyreferred to by the reference numerals 100 to 1100, respectively. Similarreference numbers will be used throughout the drawings to refer tosimilar portions of similar parts.

Screws 100–1100 are shown in use with partial segments of orthopedicimplants generally referred to by the reference numerals 150–850. Asused in this application, orthopedic implants 150–850 can includevarious types of orthopedic implants such as, but not limited to,skeletal plates and interbody spinal fusion implants. Orthopedicimplants 150–850 are preferably made of resorbable (bioresorbable)materials such as, but not limited to, cortical bone, plastics, andcomposite plastics. Suitable plastics may include those comprisinglactides, galactides, glycolide, capronlactone, trimethylene carbonate,dioxanone in various polymers and/or combinations. Implants 150–850 mayalso comprise any other material suitable for surgical implantation intothe human body including various metals, ceramics, and non-resorbableplastics and composites.

Screws 100–1100 each have a shaft that is threaded at least in partpreferably having a tip at its leading end and may have a screw head atits trailing end. The screw head may or may not be protuberant, but isadapted to prevent the forward movement of the screw once the head isfully engaged to the orthopedic implants 150–850 or an appropriateopening in a bone.

Screws 100–1100 may be further secured to the orthopedic implants150–850 by locks that are preferably made of cortical bone and may alsobe made of resorbable materials.

FIGS. 1A–1C show a screw 100 with a shaft 102 having a leading end 104,a tip 106, and a trailing end 108 opposite leading end 104. Shaft 102 isconfigured to pass through an opening 140 in an implant 150 adapted toreceive a bone screw. Shaft 102 has a thread 110 adapted to engage bone.Shaft 102 has a minor diameter and an outer diameter as measured fromthe peaks of threads 110 to the minor diameter. The turns of thread 110are spaced apart by valleys preferably having rounded bottoms. As aresult, thread 110 has a wide base. The preferred embodiments of thepresent invention have generally enlarged root diameters to increasetheir strength overall and to withstand the torques generated oninsertion which may be further mitigated by the prior use of a tap.Further stress risers are avoided by avoiding sharp corners andunradiused edges. Further, it may be desirable to have the root diameterbe generally increased nearer to the head end of the screws. Thread 110preferably has a self-tapping first turn near tip 106.

As shown in FIG. 1D, in another embodiment screw 100′ has a thread neartip 106′ in the same configuration of thread 110 and has a threadportion 114 with a wider base such that the valleys between thread 114are more rounded than the valleys between the portion of thread 110′.This results in a screw having a shaft 102′ with wider root diameterproximate trailing end 108′ for increased strength of screw 100′.Trailing end 108 preferably has a screw head 120 with an enlargedportion having a diameter greater than the outer diameter of thethreaded portion of shaft 102 and greater than opening 140 in implant150 so as to prevent passage therethrough.

As shown in FIGS. 1A and C, screw head 120 preferably has a cruciaterecess 122 for receiving distal end 152 of screw driver 156. Bone screw100 is prevented from backing out by screw lock 130 that threads intothreaded portion 118 of opening 140 in implant 150. Screw lock 130 has atool receiving recess 132 and opening 134 for engaging lock engagementend 154 of screw driver 156 for installing screw lock 130. Distal end152 of screw driver 156 is preferably configured to pass through opening134 when engagement end 154 cooperatively engages recess 132.

As shown in FIG. 1A, in use distal end 152 is moved into opening 140 andscrew driver 156 is utilized to insert bone screw 100 by cooperativelyengaging a cruciate recess 122 in bone screw head 120. Once screw 100 isinserted into opening 140, the enlarged portion of the head 120 is atleast partially blocked from passing through implant 150 by a retainingflange 116. In a preferred embodiment, lock 130 is machined out ofcortical bone and may best be harvested from portions of the calvariumas it is less curved than a long bone. Alternatively, lock 130 couldalso be machined out of a long bone. As with other screw locks shownherein, by way of example only and not limitation, the locks canalternatively be formed of a bioresorbable plastic. Screw lock 130 ispreferably made of one cortical thickness, e.g., longitudinally along along bone, rather than by cutting transversely across the bone.

Lock 130 is preferably circumferentially threaded with threads 136 asshown in FIG. 1B. When screw driver 156 is advanced to insert lock 130into implant 150, threads 136 of lock 130 can be threaded into receivingthreads 118 of implant 150. In this example, it can be appreciated thatopening 140 need not be threaded along its entire depth. Lock 130 can berigidly tightened against the unthreaded portion of opening 140, whichacts as a stop, preventing any further movement of lock 130 into opening140. By binding lock 130 to trailing wall 112 of implant 150, and makingthe lower surface of lock 130 concave, allowance is made for motion ofscrews 100 relative to implant 150. This allows the surgeon some freedomof choice in positioning screws 100 and in selecting the direction ofthe force vector to be generated relative to implant 150.

Alternatively, by any number of structural configurations, such as forexample an interference fit between screw head 120 and implant opening140, or by way of more deeply threading opening 140, or by flatteningthe top of the screws and making the circumferential perimeter flush tolock 130, or by allowing lock 130 to contact screw head 120, latermotion of screw 100 can be prevented. Stated differently, while thepresent example shows how to allow for variability in the screw'splacement and provides for later movement of the screw as might occurwith settling, in the alternative, the path of screw 100 through implant150 can be rather narrowly defined, and any angular motion of screw 100relative to implant 150 can be prevented.

As shown in FIGS. 2A and 2B, bone screw 200 has a threaded shaft 202, aleading end 204, a tip 206, and an opposite trailing end 208. Shaft 202has a thread 210 adapted to engage bone. Trailing end 208 has a screwhead 220 having a slightly enlarged head portion. Head portion 220 has adimension greater than the outer diameter of the thread portion 210 ofshaft 202 and greater than opening 240 in the implant 250 so as toprevent passage therethrough. Screw head 220 has a hex-shaped perimeter222 adapted to complimentary engage a hex socket driver for installingthe screw. A screw lock 230 having a threaded shaft 236 and an enlargedhead 238 is used to lock screws 200 to implant 250 and to prevent themfrom backing out. Screws 200 are preferably non-parallel to thelongitudinal axis of implant 250, more preferably at an angle of between25° and 75° relative to the implant longitudinal axis. Lock 230 includesa hexagonal tool receiving area 232 for rotatably inserting lock 230 inimplant 250. Screws 200 are adapted to angle toward each other in thedirection of the arrows in FIG. 2A.

As shown in FIGS. 3A and 3B, each one of bone screws 300 has a threadedshaft 302 having a leading end 304, a tip 306, and an opposite trailingend 308. Shaft 302 has a thread 310 for engaging bone. Trailing end 308has a screw head 320 having an enlarged portion with a diameter greaterthan the outer diameter of the threaded portion of shaft 302 and greaterthan opening 340 in implant 350 so as to prevent passage therethrough.Screw head 320 has a cross slotted cruciate recess 322 similar to thatshown in FIG. 1C for receiving end 152 of screw driver 156. Bone screw300 is prevented from backing out by screw lock 330 that threads intothreaded opening 318 of implant 350. The bone screw lock 330 has a toolreceiving recess 332 for engaging end 352 of a screw driver forinstalling the screw lock 330. Screw lock 330 may also have cutawayportions 360 and 362 to permit the insertion of bone screws 300 intoopenings 340 while screw lock 330 is attached to implant 350 in anunlocked position. Screw lock 330 can be rotated to a locked position tocover at least a portion of screw heads 320 and lock bone screws 300 toimplant 350.

FIG. 3A shows lock 330 in use, where it can be appreciated that headportion 320 of screw 300 is prevented from passing through implant 350by a retaining flange 316 at the base of opening 340. It can also beappreciated that when lock 330 is fully tightened, a lower portion ofthe screw head can be tightened against implant 350 itself so as to, aspreviously described, allow for some convergent motion of the bonescrews. FIG. 3B shows lock 330 having a hex well 332 therein, andopposed concave portions 360 and 362. It should be understood thatvarious driver engaging structures are useful for the intended purposeare contemplated and within the scope of the present invention.

In a preferred embodiment, when lock 330 is one-quarter turn short ofbeing fully tightened, openings 340 are open and unobstructed at lock330 to permit bone screws 300 to be inserted therein. After screws 300have been installed, lock 330 can be further tightened by turning it 90degrees until the shaft of the lock 330 reaches the lower most internalthreads 318, thereby allowing the locking screw to be solidly tightenedto implant 350.

While screws 300 have freedom to move closer together, screws 300 cannotback out of implant 350 with lock 330 in place. As previously discussed,while this is considered preferable, implant 350 can be so constructedto prevent any angular freedom of screw 300 relative to implant 350.Further, implant 350 and lock 330 can be configured to cooperate toprevent any backward motion of screw head 320.

As shown in FIG. 4, screw 400 has a threaded shaft 402 having a leadingend 404, a tip 406, and an opposite trailing end 408. As with all of theembodiments described herein, tip 406 may have various configurationsand may or may not have cutting flutes. Such flutes may be useful whensuch screws are for insertion into cancellous bone. However, when thebone is dense, the use of a thread forming tap as could be made of aninstrument quality metal would be preferable. The tap would cut the pathfor the screw to follow rather than have the screw be subjected to theforces that would be required to form thereafter.

Shaft 402 has a thread 410 for engaging bone. Trailing end 408 has ascrew head 420 having an enlarged portion having a diameter greater thanthe outer diameter of the threaded portion of shaft 402 and greater thanopening 440 in implant 450 so as to prevent passage therethrough. Bonescrew 400 is prevented from backing out by screw lock 430. Lock 430 isin the form of a disc with a threaded side wall 462 capable ofthreadably engaging threads 464 within common opening 460. When screws400 are locked to implant 450, screws 400 can be allowed some angularmotion relative to implant 450. The screw lock 430 has a tool receivingrecess for engaging the end of a screw driver for installing the screwlock 430. Screw lock 430 and implant 450 are preferably made of corticalbone.

Distal to heads 420 of screws 400 is a smooth shaft portion 458 of alesser cross sectional dimension than opening 440 which, in combinationwith the available space within common opening 460 between screw head420 and lock 430, allows for bone screw 400 to operate as a lag screw,but, nevertheless, be capable of some variability in its positioning andto still further be capable of some ability to move closer to implant450.

FIG. 5 shows an alternative implant screw and lock arrangement for usewith implant 550, or as with the lock and screw configuration of FIG. 4with any of the other embodiments of the present invention as may beappropriate. To that end, it should be appreciated that the implantsshown herein are by way of example only and without limitation to thevarious combinations and permutations of the various screw, lock, andimplant configurations shown, as well as the substantial equivalentthereof which are within the scope of the present invention.

As shown in FIG. 5, screw 500 has a threaded shaft 502 having a leadingend 504, a tip 506, and an opposite trailing end 508. Shaft 502 has athread 510 for engaging bone. Trailing end 508 has a screw head 520having an enlarged portion having a diameter greater than the outerdiameter of the threaded portion of shaft 502 and greater than opening540 in implant 550 so as to prevent passage therethrough. Screw head 520is preferably adapted to cooperatively engage the end of screw driver.Bone screw 500 is prevented from backing out by screw lock 530 that hasa shaft 536 that threads into internal threads 518 implant 550. As withother embodiments herein described, the screw lock 530 may have a toolreceiving recess for engaging an end of a screw driver for installingscrew lock 530. Screw 500 and lock 530 are preferably made of corticalbone as preferably is implant 500. The combination results in screw 500having a precise and defined orientation that is further secured by lock530 so that no angular deviation of the screw in the construct isallowed.

Lock 530 differs from lock 430 in that extending from head portion 520is a threaded shaft 536 for threading into internal threads 518 betweenopposed openings 540 within common opening 560 of implant 550. Unlikethe mechanism illustrated in FIG. 4 where cap 430 tightens against theinternal implant wall rather than only pressing against the screw headsthemselves, thereby permitting some motion, head 538 of lock 530tightens against heads 520 of screws 500 which also differ from screws400 in that the smooth proximal shaft portions 558 are adapted to forman interference fit with the passageway through implant 550 and tothereby in combination allow for the screws to have a precise trajectoryand to further be rigidly locked to the implant. It should beappreciated then that FIGS. 4 and 5 each teach a structure by which animplant of the present invention can be constructed so as to eithercause the screws passing therethrough to have a fixed trajectory or inthe alternative, be capable of variable angle placement. Further taughtis structure for permitting the present invention implants to eitherallow for, or to prevent post-deployment angular motion of the bonescrews relative to the implant after the screws have been locked withinthe implant; or still further to allow for but one degree of freedom ofthe locked screws and that being only to allow for the settling, or thecoming closer together of the adjacent vertebrae. Various ways forachieving each of these are shown herein and may be combined in variousways with various embodiments of the implants shown or their substantialequivalents without departing from the scope of the present invention.

As shown in FIG. 6, screw 600 has a threaded shaft 602 having a leadingend 604, a tip 606, and an opposite trailing end 608. Shaft 602 has afirst thread 610 for engaging bone. Trailing end 608 has an enlargedportion 620 having a diameter greater than the outer diameter of thethreaded portion of shaft 602 and greater than opening 640 in theimplant 650 so as to prevent passage therethrough. Opening 640 ispreferably threaded. Proximate enlarged portion 620 is a second thread612 having a different thread pitch than thread 610. For example, thread610 could have a thread pitch similar to a wood screw and thread 612could have a thread pitch similar to a metal screw. Screws 600 are selflocking as a result of the different thread pitches of screw 600 incombination with thread opening 640. That is the combination of screw600 with implant 650 adapted to receive that screw, obviates the needfor a third element such as a lock, and functions to achieve locking bythe interaction of the two elements.

As shown in FIGS. 7A and 7B, screw 700 has a threaded shaft 702 having aleading end 704, a tip 706, and an opposite trailing end 708. Shaft 702has a first thread form 710 for engaging bone. Trailing end 708 has anenlarged portion 720 having a diameter greater than the outer diameterof the threaded portion of shaft 702 and greater than opening 740 inimplant 750 so as to prevent passage therethrough. Opening 740 ispreferably threaded at least in part. Enlarged portion 720 has a secondthread 712 having a different thread pitch than thread 710. For example,thread 710 could have a thread pitch similar to a wood screw and thread712 could have a thread pitch similar to a metal screw.

Unlike screw 600, screw 700 does not have an additional enlarged headportion such as portion 620 of screw 600, but rather relies on flangeportion 716 of opening 740 to stop the further progression of the screwhead 720 through the implant and to allow for head 720 to be securelytightened to the trailing end of implant 750. Screw head 720 canpreferably have a cruciate recess 722 for receiving the end of screwdriver.

FIGS. 8–10 show an alternative embodiment of a locking screw mechanismwhich can be adapted for use with various of the other shown and/ordescribed implant embodiments and other interbody spinal fusion implantsof the present invention. Portion 808 of implant 850 has an opening 840for accepting a bone screw 800. Bone screw 800 has a head portion 820having at least in part about its perimeter a convex surface 872 havinga maximum diameter. Bone screw receiving opening 840 has acircumferential concavity 874 for receiving convexity 872 of bone screwhead 820. Bone screw head 820 has threads 880 and a plurality of slots876, preferably two to four. Slots 876 allow locking screw 830 tocooperatively engage and be driven with a driver and allow for head 820to sufficiently compress to be fully received within opening 840 ofimplant 850.

As can be appreciated from FIGS. 8–10, screw 800 can be placed at anangle to implant 850. Once bone screw 800 has been fully engaged into abone such as an adjacent vertebral body, for example, the screw can befurther rotated, allowing the vertebral body to be lagged to implant850, increasing the compressive load. Once screw 800 has been properlyplaced and tightened to the extent desired by the surgeon, a lockingscrew 830 having a head 838 and a threaded shaft 836 may be threadedinto the threaded interior of bone screw head 820 via interior threads880.

The implant screw locking system of FIGS. 8–10 can be manufactured suchthat while the locking screw 830 may be lockably tightened to bone screw800, and thus the backward migration of 800 from implant 850 prevented,the system can be designed so as to either allow for angular motionafter locking screw 830 is locked to bone screw 800 or to prevent it.The function of bone screw head 820 in its ability to rotate andangulate within implant 850 is not dissimilar to the above describedvariation of the self-locking screw.

Also shown in FIGS. 8–10 are the degrees of sharpness of the arcuateportion of thread 810, or decreasing thread pitch along shaft 802towards trailing end 808. Beginning from the leading end 804 andextending towards trailing end 808, thread 810 progressively becomesthicker until at, for example, location 882 thread 810 is thicker at itsouter diameter than at location 884. The thickening increases moretowards trailing end 808. By having a sharp thread pitch 884 nearleading end 804 and a progressively less sharp thread pitch extendingtowards the trailing end, screw 800 embeds itself more securely. Threadportion 882 expands the V-cut left by the sharper thread portion 884,thereby adding force vectors substantially parallel to the screwlongitudinal axis and further tightening the fit of screw 800 within abone.

FIGS. 11–13 show a screw 900 having a threaded shaft 902 with leadingend 904, tip 906, and an opposite trailing end 908. Shaft 902 has athread 910 for engaging bone. Trailing end 908 has a screw head 920having an enlarged portion with a diameter greater than the rootdiameter of shaft 902. Screw head 920 preferably has a cruciate recess922 for receiving an insert for spreading apart cruciate recess 922 toexpand enlarged portion 920 to a dimension larger than the outerdiameter of thread 910. Recess 922 is adapted to cooperatively engageprojection 886 of screw driver end 952. Engaging projection 886 extendsalong the longitudinal axis of screw driver 956. In use, engagingprojection 886 will slidably engage with recesses 922 and screw head920. The extended length of engaging projection 886 allows the surgeonto more forcefully and securely apply a rotative force upon screw 900.Although a cruciate or “plus sign” configuration is illustrated forengaging projection 886, a wide range of alternatives are available andcontemplated by the scope of the present invention.

Tool 956 may be manual or powered. Tool 956 preferably has a push-downinner shaft connected to engaging projection 886 so that in use, thesurgeon will push down on tool 956 and rotate to insert screw 900.Alternatively, if tool 956 is electrically powered, an inner shaftconnected to engaging projection 886 may be made to rotate upon thesurgeon pushing down on the tool after proper positioning. Although atool with a push down inner shaft is preferred, it is not required. Forexample, engaging portion 886 may be fixedly attached within tool 956.

FIG. 16 shows an embodiment of the present invention with implant 550′properly implanted across the disc space between adjacent vertebralbodies V₁ and V₂. Openings 588′ of implant 550′ allow for vascularaccess through trailing end 512′ of implant 550′ and for bone growththerethrough. Trailing end 512′ has common opening 560′ and situatedessentially therein, is threaded opening 318′ for receiving an implantdriver. The implant driver has a distal end for a complimentary fitwithin common opening 560′ and therethrough a rotatable threaded memberfor threading into opening 318′. Internal threads 318′ of implant 550′are configured to receive lock 530′ in a way similar to that illustratedin FIG. 5. Opening 560′ also is adapted to receive a screw device tolink the implant to other implants, to a staple, or to receive a lockingscrew to lock bone engaging screws to the implant as disclosed inMichelson U.S. patent application Ser. No. 08/926,334 incorporatedherein by reference. Common opening 560′ also may have therein opposedand divergently angled openings 540′ adapted to receive opposedvertebral bone engaging screws 500′. Bone screw receiving openings 540′preferably may have circumferentially around them retaining seats 516′adapted to receive and to block the passage of the heads of screws 500′to be inserted therethrough. Retaining seats 516′ may also be flanged.

FIG. 17 shows screws 600′ of the present invention used with a plate692′ and an implant 690′ inserted into the disc space between adjacentvertebral bodies V₁, V₂. Preferably, both plate 629′ and implant 690′are formed of bone. Screws 600′ may have a dual-threaded shaft. That is,the leading end may be threaded for engaging cancellous bone while thetrailing end may be separately threaded for engagement with bone plate692′. Preferably, an unthreaded shaft portion 658′ is between thethreaded shaft portions. In this embodiment, flange 618′ in plate 692′will act as a stop to the enlarged portion of screw head 620′.

FIG. 18 shows a fractured bone screwed together with lag screws 1000having different ratios of threaded to non-threaded shaft lengths. Thesurgeon may select screws based on the depth of the bone and thelocation of the fracture. By selecting screws with a non-threaded shaftlength 1058 near trailing end 1008 to match the first, proximatefractured bone portion, the surgeon can insert substantially all thethreaded shaft length 1002 at the leading screw end 1004 into thesecond, more distal fractured bone portion. In a manner similar toembodiments described before, an enlarged portion of screw head 1020cooperates with the outer surface of the bone to assist in locking thefirst, proximate bone portion with the second, distal bone portion.Selecting screws of varying ratio between threaded and non-threadedlength to approximate the facture line will result in a more secure fitand ultimately enhance the bone healing process. The placement of smoothportions of screws 1000 in one fractured portion and threaded portionsin the other fractured portion permits the pulling together or “lagging”of the two fractured bone portions.

FIGS. 21 and 22 show a cortical bone screw 1100 having a shaft 1102 withthread 1110 configured with both a concavedly arcuate portion 1194 and aconvexedly arcuate portion 1196 along a transverse cross section to thelongitudinal axis of the screw. Preferably, concave and convex portions1194, 1196, respectively, are opposite each other as shown in FIG. 22.The concave and convex portions may be formed by cutting a strip A froma long bone as shown in FIG. 20 with a trephine having a diametergreater than the cortical thickness of the bone. The concave portion cancorrespond to the area of bone facing the medullary canal. Screw head1120 may also have concave and convex portions aligned with those ofshaft 1102. The radii of the concave portions may vary along the lengthof the longitudinal axis. The radii of each concavedly arcuate portion1194 and convexedly arcuate portion 1196 may also vary in relation toeach other along a plane transverse to the longitudinal screw axis.Although screw 1100 may be made from many different materials, it ispreferred that screw 1100 is formed substantially of cortical bone of asingle cortical thickness. Screw 1100 may also include a recess 1122 forengagement with a tool such as one hereinbefore described.

For the embodiments of bone screws described herein, by way of example,the bone screws of the present invention for use in the lumbar spinewould have an outer diameter in the range of approximately 4 to 8.5 mm,with approximately 5 to 7.5 mm being preferred; and an overall length inthe range of approximately 20 to 40 mm, with approximately 25 to 30 mmbeing preferred.

For use in the cervical spine, an outer diameter in the range ofapproximately 3 to 6 mm, with approximately 4 to 5 mm being preferred;and a length in the range of approximately 10 to 20 mm, withapproximately 12 to 16 mm being preferred.

By way of example, a bone screw of the present invention for use in thelumbar spine could have a root diameter of approximately 5 mm and anouter diameter of approximately 7.5 mm; a thread pitch of approximately3 mm distally and at the trailing end by use of a multipoint lead, atriple wound thread having a pitch of approximately 1 mm. Such a screwcould be threaded through an opening in a material having a thicknessless than the 3 mm of the distal screw pitch and an opening greater thanthe root diameter, but less than the outer diameter, until such screwwas threaded into the point where the three point lead at the trailingend of the screw engaged the threaded opening in the material. At whichpoint, the screw would be lockably engaged into the material as the wallthickness of the material into which the screw was being threaded wouldno longer fit between the individual threads of the machine threadedportion of the screw.

Alternatively, the pitch of the screws could progressively decreasetowards the trailing end, or the threads thicken to the same effect. Ina further embodiment of the present invention, the head of the screwcould be slightly flared, with or without expansion/compression slotstherethrough, such that when the head is tightened into the implant, thehead is compressed and wedged into a high interference fit to theimplant stopping the further forward motion of the screw and locking ittherein.

These screws may be cut from a single thickness of cortex as distinctfrom cutting transversely across a long bone so as to include asignificant hollow within the screw as a result of the medullary canalof that long bone.

Referring to FIGS. 19, 20 and 23, in a preferred method of manufacturingthe bone screws of the present invention, bone is harvested sterilelyfrom a donor such as a human cadaver for example, and furthermanufactured using sterilized machinery.

Referring to FIGS. 19 and 23, in a first step at least a diaphysealportion of a large tubular bone or a portion of a generallyintramembraneously formed bone such as the calvarium is removed from ahuman cadaver preferably, but not necessarily, in a sterile manner.

As illustrated in FIG. 20, in a second step the harvested bone isfurther divided longitudinally along the long axis of the large bone andpreferably, but not necessarily, into strips having a width generally asgreat or slightly greater than the thickness of the cortical bone. Forexample, strips A and B may have the configuration shown in FIGS. 19 and20. While the bone can be cut using any appropriate type of cuttingdevice such as a saw, laser, water jet, etc., a hollow tubular cuttersuch as a trephine having an inside diameter sized to match the outerdiameter of the screw to be made or greater, the thickness of the donorcortex, or slightly greater than the thickness of the donor cortex isparticularly beneficial as it provides for a generally more or lesscircular cross section that facilitates the further machining of thethreads therefrom. It should be understood that the bone diaphysis canbe cut to length prior to or after the forming of the longitudinalstrips.

In a third step, a machine appropriate for machining a screw such as alathe, a Swiss milling machine, a CNC, a thread whirling machine, orsimilar device is used to machine the screw out of the cortical bone tospecific dimensions. The bone screw is then inspected for structuralintegrity and dimensional tolerances.

It should be appreciated that while screws of cortical bone may haveconformations similar to prior art screws made of metal withoutdeviating from the teachings of the present invention, in the preferredembodiments, consideration has been given to the considerabledifferences in the material properties of these very differentmaterials. Cortical bone as a material, while much stronger thancancellous bone, is profoundly weaker than the metals in common usetoday for the fabrication of screws such as stainless steels andtitanium alloys. To that end, the preferred embodiments of the presentinvention have generally enlarged root diameters to increase theirstrength overall and to withstand the torques generated on insertionwhich may be further mitigated by the prior use of a tap. Further stressrisers are avoided by avoiding sharp corners and unradiused edges.Further, it may be desirable to have the root diameter be generallyincreased nearer to the head end of the screws. The root diameter mayprogress from tip to head, flair beneath the head, or otherwiseincrease. The root diameter should ideally flow into the head ratherthan having a sharp step off. The thread itself should be kept strong bynot unduly extending the height of the thread relative to the width ofthe base, or thinning the profile of the thread. The valleys between theturns of the thread are again preferably rounded rather than notched.Similarly the driver engaging area in the head portion of the screwshould be kept substantial and have rounded rather than square endedslots where slots are used.

In a fourth step, the screw of cortical bone may be frozen for storageuntil use. Measures are provided for providing for the storability ofthe screws such as freezing, freeze drying.

In another, the donor is tested for communicable diseases to assure thesafety of the bone. Alternatively, donor testing may be performed priorto the steps of harvesting bone from the donor.

In a sixth step, cultures are taken during the above procedure, butprior to the bone being frozen, to assure the safety of the bone.

In an alternative method of manufacturing the present invention,sterility may or may not be a goal during the manufacturing of the screwand the screw is subjected to a sterilization process including, but notlimited to, exposure to radiation, freeze drying, denaturing, cleaning,chemical sterilization including the use of sterilizing liquids and/orgases.

In a further step in the manufacture of the screws of the presentinvention, the screw may be immersed in, or coated with, biologicallyactive chemical agents included, but not limited to, antibiotics, orsubstances to promote bone formation such as bone morphogeneticproteins, mineralizing proteins, hydroxyapatite, or genetic materialcoding for the production of bone (directly or indirectly).

In a further step in the manufacturing process which may occur before orafter freezing, or if not frozen then before or after sterilization thescrew is packaged. In a preferred embodiment, the sterile screw iscontained within a sterile container, which is itself contained withinan internally sterile second container.

In a further step, each screw package comprises identifying informationsuch as a tracking number by which the donor can be identified if needbe, the identity of the manufacturer, the date of expiration by whichthe screw must be used, and the length and at least nominal outsidediameter of the screw and if frozen the requirements for proper storageof the screw. It should be appreciated that the present invention is notlimited by the specific form of the thread, and includes both machineand wood type threads. Also, the present invention does not require thatthe thread be fully continuous and/or of constant height. For example,where it is desirable to form a screw of the largest possible maximumoutside diameter, rather than uniform outside diameter it may bedesirable to then form the screw from a longitudinal diaphyseal stripselected to be wider than it is thick such that when machined theresultant screw will have a root diameter that is circular in crosssection and an outside diameter that in cross section will have opposedarcuate portions, but that is incompletely circular.

It is appreciated that the present invention is not directed just tobone screws, but also to the locks themselves made of bone for lockingthe bone screw which may also take the form of a threaded member or evenof a screw itself. The locks and bone screws of the present inventionmay be combined with various orthopedic and spinal plates and interbodyspinal fusion implants. The implants themselves, that is the plate orthe fusion implant are preferably also made of a bioresorbable materialincluding, but not limited to, human cortical bone.

It is appreciated that bone screws of the present invention are formedalong the longitudinal axis of a long bone or from a single cortexrather than being formed by cutting transversely through a long bone soas to include the opposite cortices of a long bone, that is both thenear and far cortices of a tubular bone, as well as the side walls. Thescrews and locks of the present invention are a manufactured device thatwould essentially be a machined orthopedic screw, machined from a singlelength of cortex if of a long bone or from the thickness of anintramembraneously formed bone such as calvarium. The present inventionis directed to a bone screw made substantially of cortical bone havingan insertion end, a shaft at least in part threaded and a head havingeither a greater dimension than the outer diameter of the threaded shaftor functional means to resist the continuing forward advancement of thescrew when properly utilized. By way of example, a screw made ofcortical bone without a protuberant head and a thread pitch that is moretightly wound at its trailing end than at its leading end. The presentinvention is also specifically directed to bone screws that are nothollow through the shaft portion of the screw that are madesubstantially of cortical bone and which is machined and manufactured.

Moreover, the configuration of the thread of the screws is not limitedto the configuration shown in the drawings but may also have any otherconfiguration suitable for its intended purpose.

While the present invention has been described with respect to thepreferred embodiments, it is appreciated that variations of theembodiments are possible without departing from the scope of the presentinvention.

1. A screw for use in the human body formed of cortical bone from amajor long bone having a medullary canal, said screw comprising aleading end, a trailing end opposite said leading end, and a shafttherebetween, said shaft having a mid-longitudinal axis, a length, and athread extending from said shaft along at least a portion of its length,said shaft having a cross section transverse to said mid-longitudinalaxis through said thread having a concavedly arcuate portion and aconvexedly arcuate portion opposite said concavedly arcuate portion,said concavedly arcuate portion being formed from at least a portion ofthe medullary canal and extending along a majority of the length of saidscrew, said cross section bisecting a rotation of said thread.
 2. Thescrew of claim 1, wherein said cross section has opposite convexportions with approximately the same radius, said concavedly arcuateportion and said convexedly arcuate portion being between said oppositeconvex portions, said convexedly arcuate portion having a radius greaterthan the radius of said opposite convex portions, said cross sectionbeing through said concavedly arcuate portion of said thread.
 3. Thescrew of claim 1, wherein said trailing end is configured tocooperatively engage at least a portion of the screw hole of an implantso as to prevent said screw from linear motion along themid-longitudinal axis of said shaft in a direction opposite to thedirection of insertion when said screw is threaded through a screw holeto attach the implant to a bone portion of the human body.
 4. The screwof claim 1, wherein said screw is formed substantially of cortical boneof a single cortical thickness.
 5. The screw of claim 4, wherein saidcortical bone is obtained from a human.
 6. The screw of claim 4, whereinsaid cortical bone is obtained from a generally intramembraneouslyformed cortical bone.
 7. The screw of claim 4, wherein said corticalbone is obtained from a large tubular bone of a human.
 8. The screw ofclaim 7, wherein said cortical bone is from the diaphyseal region ofsaid large tubular bone.
 9. The screw of claim 7, wherein the tubularbone is a femur.
 10. The screw of claim 1, further comprising anenlarged portion proximate said trailing end with a dimension transverseto the mid-longitudinal axis of said shaft greater than said outerdiameter of said thread, said enlarged portion configured to preventsaid trailing end from passing through the screw hole in the implant.11. The screw of claim 10, wherein said enlarged portion forms a head.12. The screw of claim 10, wherein said enlarged portion forms a lip.13. The screw of claim 1, wherein said trailing end includes a secondthread having a different thread pitch than said thread along saidshaft.
 14. The screw of claim 13, wherein the thread pitch of saidsecond thread is similar to a metal screw pitch.
 15. The screw of claim13, wherein the thread pitch of said thread along said shaft is similarto a wood screw pitch.
 16. The screw of claim 1, wherein the threadpitch of said thread along said shaft is similar to a wood screw pitch.17. The screw of claim 1, wherein at least a portion of said trailingend is expandable.
 18. The screw of claim 17, wherein at least a portionof said trailing end is divided into at least two members with anopening therebetween.
 19. The screw of claim 18, further comprising aninsert configured to fit into said opening of said trailing end and tomove said at least two members apart when inserted into said opening.20. The screw of claim 19, wherein said insert is configured to beinserted by linear advancement into said opening.
 21. The screw of claim20, wherein said insert has a cruciate shape and said opening has acorresponding cruciate shape.
 22. The screw of claim 19, wherein saidinsert is configured to be inserted by rotational movement into saidopening.
 23. The screw of claim 22, wherein said insert is threaded. 24.The screw of claim 1, wherein at least a portion of said trailing end isconfigured to cooperatively engage a driving instrument for insertion ofsaid screw.
 25. The screw of claim 24, wherein said trailing endincludes a recess to cooperatively engage a driving instrument.
 26. Thescrew of claim 25, wherein said recess is one of cruciate-shape andhex-shaped.
 27. The screw of claim 24, wherein said trailing endincludes a protrusion to cooperatively engage a driving instrument. 28.The screw of claim 27, wherein said protrusion has a hex-shapedperimeter.
 29. The screw of claim 1, wherein said thread is sharperproximate said leading end than proximate said trailing end.
 30. Thescrew of claim 1, wherein said thread has a V-shaped cross section withan apex and a base adjacent to said shaft, said base being substantiallywider than said apex.
 31. The screw of claim 1, wherein said thread hasa peak as measured from said shaft, the peak being greater proximatesaid leading end than said trailing end.
 32. The screw of claim 1,wherein said shaft has a root diameter that increases in the directionfrom said leading end to said trailing end.
 33. The screw of claim 1,wherein said leading end forms a tip and said tip is fluted.
 34. Thescrew of claim 1, further comprising a bioresorbable material other thancortical bone.
 35. The screw of claim 34, wherein said material includesbioresorbable plastics.
 36. The screw of claim 34, wherein said materialincludes at least one of glycolide polymers, lactide, capralactone,trimethylene carbonate, and dioxanone.
 37. The screw of claim 1, whereinsaid screw comprises bone growth promoting material.
 38. The screw ofclaim 37, wherein said bone growth promoting material is selected fromone of bone morphogenetic protein, hydroxyapatite, and genes coding forthe production of bone.
 39. The screw of claim 1, wherein said screw istreated with a bone growth promoting substance.
 40. The screw of claim1, in combination with a bone growth promoting material.
 41. The screwof claim 40, wherein said bone growth promoting material includes atleast one of bone morphogenetic protein, mineralizing proteins,hydroxyapatite, and genetic material coding for the production of bone.42. The screw of claim 1, in combination with an instrument forinserting said screw.
 43. The screw of claim 1, wherein said thread hasa plurality of turns, a majority of said turns including said concavedlyarcuate portion.
 44. The screw of claim 43, wherein all of said turnsinclude said concavedly arcuate portion.
 45. The screw of claim 43,wherein the radii of at least two of said concavedly arcuate portionsvary along the length of said shaft.
 46. The screw of claim 1, whereinsaid thread has an outer diameter that is generally constant along thelength of said shaft.
 47. A screw formed from a major long bone having amedullary canal, said screw comprising a leading end, a trailing endopposite said leading end, and a shaft therebetween, said shaft having amid-longitudinal axis, a length, and a thread extending from said shaftalong at least a portion of its length, said shaft having a crosssection transverse to said mid-longitudinal axis through said threadhaving a concavedly arcuate portion and a convexedly arcuate portionopposite said concavedly arcuate portion, said concavedly arcuateportion being formed from at least a portion of the medullary canal andextending along a majority of the length of said screw, said crosssection bisecting a rotation of said thread, said screw being formed bythe process of cutting a strip of cortical bone having a single corticalthickness from the long bone in the direction of the longitudinal axisof the long bone and machining said strip to form a thread.
 48. Thescrew of claim 47, wherein said strip of cortical bone is cut with atrephine having a diameter greater than the cortical thickness of thelong bone.
 49. The screw of claim 48, wherein said cross section hasopposite convex portions with approximately the same radius, saidconcavedly arcuate portion and said convexedly arcuate portion beingbetween said opposite convex portions, said convexedly arcuate portionhaving a radius greater than the radius of said opposite convexportions, said cross section being through said concavedly arcuateportion of said thread.
 50. The screw of claim 47, in combination with abone growth promoting material.
 51. The screw of claim 50, wherein saidbone growth promoting material includes at least one of bonemorphogenetic protein, mineralizing proteins, hydroxyapatite, andgenetic material coding for the production of bone.
 52. The screw ofclaim 47, in combination with an instrument for inserting said screw.53. The screw of claim 47, wherein said thread has a plurality of turns,a majority of said turns including said concavedly arcuate portion. 54.The screw of claim 53, wherein all of said turns include said concavedlyarcuate portion.
 55. The screw of claim 53, wherein the radii of atleast two of said concavedly arcuate portions vary along the length ofsaid shaft.
 56. The screw of claim 47, wherein said thread has an outerdiameter that is generally constant along the length of said shaft. 57.A method for forming a screw made of cortical bone, comprising the stepsof: cutting a strip of cortical bone having a single cortical thicknessfrom a long bone in the direction of the longitudinal axis of the longbone; and machining said strip to form a screw having a shaft with amid-longitudinal axis, a length, and a thread extending from said shaftalong at least a portion of its length, said shaft having a crosssection transverse to said mid-longitudinal axis through said threadhaving a concavedly arcuate portion and a covexedly arcuate portionopposite said concavedly arcuate portion, said cross section bisecting arotation of said thread.
 58. The method of claim 57, wherein the cuttingstep includes the sub-step of using a trephine having a diameter greaterthan the cortical thickness of the long bone.
 59. The method of claim58, wherein the machining step includes the sub-steps of forming saidcross section with opposite convex portions having approximately thesame radius, said concavedly arcuate portion and said convexedly arcuateportion being between said opposite convex portions, said convexedlyarcuate portion having a radius greater than the radius of said oppositeconvex portions, said cross section being through said concavedlyarcuate portion of said thread.
 60. The method of claim 57, furthercomprising the step of coating the screw with at least one of bonemorphogenetic protein, mineralizing proteins, hydroxyapatite, andgenetic material coding for the production of bone.
 61. A screw for usein the human body formed of cortical bone from a major long bone havinga medullary canal, said screw comprising: a leading end and a trailingend opposite said leading end; a shaft between said leading and trailingends, said shaft having a mid-longitudinal axis, a length, and a threadextending from said shaft along at least a portion of its length, saidthread having an outer diameter, said shaft having a cross sectiontransverse to the mid-longitudinal axis through said thread having aconcavedly arcuate portion and a convexedly arcuate portion oppositesaid concavedly arcuate portion, said concavedly arcuate portion beingformed from at least a portion of the medullary canal and extendingalong a majority of the length of said screw, said cross sectionbisecting a rotation of said thread; and an enlarged portion proximatesaid trailing end with a dimension transverse to the mid-longitudinalaxis of said shaft greater than the outer diameter of said thread, saidenlarged portion configured to prevent said trailing end from passingthrough a screw hole in an implant, said enlarged portion including aconcavedly arcuate portion formed from at least a portion of themedullary canal.
 62. The screw of claim 61, wherein said cross sectionhas opposite convex portions with approximately the same radius, saidconcavedly arcuate portion and said convexedly arcuate portion beingbetween said opposite convex portions, said convexedly arcuate portionhaving a radius greater than the radius of said opposite convexportions, said cross section being through said concavedly arcuateportion of said thread.
 63. The screw of claim 61, wherein said trailingend is configured to cooperatively engage at least a portion of thescrew hole of the implant so as to prevent said screw from linear motionalong the mid-longitudinal axis of said shaft in a direction opposite tothe direction of insertion when said screw is threaded through the screwhole to attach the implant to a bone portion of the human body.
 64. Thescrew of claim 61, wherein said screw is formed substantially ofcortical bone of a single cortical thickness.
 65. The screw of claim 64,wherein said cortical bone is obtained from a human.
 66. The screw ofclaim 64, wherein said cortical bone is obtained from a generallyintramembraneously formed cortical bone.
 67. The screw of claim 64,wherein said cortical bone is obtained from a large tubular bone of ahuman.
 68. The screw of claim 67, wherein said cortical bone is from thediaphyseal region of said large tubular bone.
 69. The screw of claim 67,wherein the tubular bone is a femur.
 70. The screw of claim 61, whereinsaid enlarged portion forms a head.
 71. The screw of claim 61, whereinsaid enlarged portion forms a lip.
 72. The screw of claim 61, whereinthe thread pitch of said thread along said shaft is similar to a woodscrew pitch.
 73. The screw of claim 61, wherein at least a portion ofsaid trailing end is configured to cooperatively engage a drivinginstrument for insertion of said screw.
 74. The screw of claim 73,wherein said trailing end includes a recess to cooperatively engage adriving instrument.
 75. The screw of claim 61, wherein said thread issharper proximate said leading end than proximate said trailing end. 76.The screw of claim 61, wherein said thread has a V-shaped cross sectionwith an apex and a base adjacent to said shaft, said base beingsubstantially wider than said apex.
 77. The screw of claim 61, whereinsaid thread has a peak as measured from said shaft, the peak beinggreater proximate said leading end than said trailing end.
 78. The screwof claim 61, wherein said shaft has a root diameter that increases inthe direction from said leading end to said trailing end.
 79. The screwof claim 61, wherein said leading end forms a tip and said tip isfluted.
 80. The screw of claim 61, further comprising a bioresorbablematerial other than cortical bone.
 81. The screw of claim 80, whereinsaid material includes bioresorbable plastics.
 82. The screw of claim80, wherein said material includes at least one of glycolide polymers,lactide, capralactone, trimethylene carbonate, and dioxanone.
 83. Thescrew of claim 61, wherein said screw is treated with a bone growthpromoting substance.
 84. The screw of claim 61, in combination with abone growth promoting material.
 85. The screw of claim 84, wherein saidbone growth promoting material includes at least one of bonemorphogenetic protein, mineralizing proteins, hydroxyapatite, andgenetic material coding for the production of bone.
 86. The screw ofclaim 61, in combination with an instrument for inserting said screw.87. The screw of claim 61, wherein said thread has a plurality of turns,a majority of said turns including said concavedly arcuate portion. 88.The screw of claim 87, wherein all of said turns include said concavedlyarcuate portion.
 89. The screw of claim 87, wherein the radii of atleast two of said concavedly arcuate portions vary along the length ofsaid shaft.
 90. The screw of claim 61, wherein said thread has an outerdiameter that is generally constant along the length of said shaft. 91.A screw formed of cortical bone for use in the human body, said screwcomprising a leading end, a trailing end opposite said leading end, anda shaft therebetween, said shaft having a mid-longitudinal axis, alength, and a thread extending from said shaft along at least a portionof its length, said shaft having a cross section transverse to saidmid-longitudinal axis through said thread having a concavedly arcuateportion and a convexedly arcuate portion opposite said concavedlyarcuate portion, said cross section having opposite convex portions withapproximately the same radius, said concavedly arcuate portion and saidconvexedly arcuate portion being between said opposite convex portions,said convexedly arcuate portion having a radius greater than the radiusof said opposite convex portions, said cross section bisecting arotation of said thread and being through said concavedly arcuateportion of said thread.
 92. The screw of claim 91, wherein said trailingend is configured to cooperatively engage at least a portion of a screwhole of an implant so as to prevent said screw from linear motion alongthe mid-longitudinal axis of said shaft in a direction opposite to thedirection of insertion when said screw is threaded through a screw holeto attach the implant to a bone portion of the human body.
 93. The screwof claim 91, wherein said screw is formed substantially of cortical boneof a single cortical thickness.
 94. The screw of claim 91, wherein atleast a portion of said trailing end is configured to cooperativelyengage a driving instrument for insertion of said screw.
 95. The screwof claim 91, further comprising a bioresorbable material other thancortical bone.
 96. The screw of claim 95, wherein said material includesbioresorbable plastics.
 97. The screw of claim 95, wherein said materialincludes at least one of glycolide polymers, lactide, capralactone,trimethylene carbonate, and dioxanone.
 98. The screw of claim 91,wherein said screw comprises bone growth promoting material.
 99. Thescrew of claim 98, wherein said bone growth promoting material isselected from one of bone morphogenetic protein, hydroxyapatite, andgenes coding for the production of bone.
 100. The screw of claim 91,wherein said screw is treated with a bone growth promoting substance.101. A screw comprising a leading end, a trailing end opposite saidleading end, and a shaft therebetween, said shaft having amid-longitudinal axis, a length, and a thread extending from said shaftalong at least a portion of its length, said shaft having a crosssection transverse to said mid-longitudinal axis through said threadhaving a concavedly arcuate portion and a convexedly arcuate portionopposite said concavedly arcuate portion, said cross section havingopposite convex portions with approximately the same radius, saidconcavedly arcuate portion and said convexedly arcuate portion beingbetween said opposite convex portions, said convexedly arcuate portionhaving a radius greater than the radius of said opposite convexportions, said cross section bisecting a rotation of said thread andbeing through said concavedly arcuate portion of said thread, said screwbeing formed by the process of cutting a strip of cortical bone having asingle cortical thickness from a long bone in the direction of thelongitudinal axis of the long bone and machining said strip to form athread.
 102. The screw of claim 101, wherein said strip of cortical boneis cut with a trephine having a diameter greater than the corticalthickness of the long bone.
 103. The screw of claim 101, in combinationwith a bone growth promoting material.
 104. The screw of claim 103,wherein said bone growth promoting material includes at least one ofbone morphogenetic protein, mineralizing proteins, hydroxyapatite, andgenetic material coding for the production of bone.
 105. The screw ofclaim 101, in combination with an instrument for inserting said screw.106. The screw of claim 101, wherein said thread has a plurality ofturns, a majority of said turns including said concavedly arcuateportion.
 107. The screw of claim 106, wherein all of said turns includesaid concavedly arcuate portion.
 108. The screw of claim 106, whereinthe radii of at least two of said concavedly arcuate portions vary alongthe length of said shaft.
 109. The screw of claim 101, wherein saidthread has an outer diameter that is generally constant along the lengthof said shaft.